CI researchers work on particle accelerators which are some of the largest structures in the world – the LHC at CERN is 27 km around and there are plans for an even larger 100 km successor. But CI researchers also work on making accelerators as small as possible, which would have a huge impact on the use of accelerators for applications such as medical and industrial imaging or cancer treatment with high energy electrons.
One of the new technologies for making smaller and cheaper particle accelerators is called laser wakefield plasma acceleration. In this, a high power laser drives a wake of oscillating electrons in a plasma, like a boat leaving a wake behind it, and some of the free electrons in the plasma can ‘surf’ on the wake and reach very high energies (10s of GeV) in only a few cm. This compares to traditional accelerators made from metal cavities, which need km to accelerate particles to similar energies.
However, as the laser travels through the plasma, it gets larger in size and eventually becomes too small in intensity to drive the plasma wake the electrons need to reach high energies.
The solution to this problem has been to use hollow capillaries filled with gas. The gas is ionised by an electrical discharge which forms a plasma channel that contains and guides the high power laser over many cm. Though these have worked very successfully, as the laser spot jitters in space it can damage the guide structure and so they do not last very long.
A team of researchers, led by the University of Oxford and involving members form the Universities of Liverpool and Maryland, as well as STFC have now come up with a different solution – they have made a waveguide out of the plasma itself. The team carried out their experiment at the Gemini-Astra laser at the Central Laser Facility of the Rutherford Appleton Laboratory in Oxfordshire.
In their experiment, they used a low power laser pulse to make a narrow plasma column, smaller than a human hair, which acted like an optical fibre to guide a second high power laser pulse. Successful guiding of the focused high intensity laser pulse was shown over 100mm, over 20 times further than the small laser spot would travel by itself. Importantly, because the guide structure was made out of the plasma itself, it cannot be damaged. This technique can hence be used many times without needing solid structures and provides an indestructible waveguide.
This is the first time guiding of a high intensity laser pulse has been demonstrated over such a long distance at the plasma densities required for high gain acceleration of electrons. Dr Laura Corner from the University of Liverpool who is a member of the research team said: “This is an exciting step forward for making laser driven accelerators practical. We were very pleased to see successful guiding of the high power laser, and especially over such long distances which would enable high energy electron acceleration.” Perhaps the next generation of accelerators might be made from lasers and plasma!
More information
A. Picksley et. al., Physical Review Accelerators and Beams 23, 081303 (2020)
https://journals.aps.org/prab/abstract/10.1103/PhysRevAccelBeams.23.081303